The present invention relates generally to color imaging assemblies utilizing beam splitters to generate spatially separated, color component images of an object which are focused on a common image plane. The invention relates more particularly to beam splitters which employ dichroic surfaces for separating a polychromatic imaging light beam into a plurality of spatially separated color component beams.
The phrase "light beam" is sometimes narrowly defined as a bundle of parallel light rays such as those generated by a collimated light source. The phrase "light beam" may also be more broadly defined as any narrow shaft of light having light rays traveling in the same general direction. Used in this broader sense, the light which emanates from an object and passes through the aperture of an imaging lens as well as the converging cone of light which emerges from the lens and which is focused at an image plane may be collectively referred to as a "light beam." When the phrase "light beam" is used herein, it is to be understood that this broader meaning is intended.
Imaging devices such as color scanners, video cameras, camcorders, and the like produce data signals representative of color images of subject objects. Beam splitters are typically utilized within such imaging devices to separate a polychromatic imaging light beam from the object into three separate color component beams which are in turn used to generate data signals which are representative of color component images (typically, red, green and blue component images) of the object. Various color imaging devices and assemblies utilizing beam splitters are described in U.S. Pat. No. 4,709,144 for COLOR IMAGER UTILIZING NOVEL TRICHROMATIC BEAMSPLITTER AND PHOTOSENSOR of Vincent; U.S. Pat. No. 4,870,268 for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS of Vincent et al.; U.S. Pat. No. 4,926,041 for OPTICAL SCANNER of Boyd et al.; U.S. Pat. No. 5,032,004 for BEAM SPLITTER APPARATUS WITH ADJUSTABLE IMAGE FOCUS AND REGISTRATION of Steinle; and U.S. Pat. No. 5,040,872 for BEAM SPLITTER/COMBINER WITH PATH LENGTH COMPENSATOR of Steinle, which are each hereby specifically incorporated by reference for all that is disclosed therein. A conventional color imaging assembly such as that shown in U.S. Pat. No. 5,040,872 will now be described with reference to FIGS. 1-3.
As shown in FIG. 1, the color imaging assembly 10 may comprise an imaging lens 12 adapted for receiving a polychromatic imaging light beam 14 emanating from an object (not shown), which in most color scanners is a line object or "scan line". The color imaging assembly 10 may also comprise a multilayered dichroic beam splitter 20 disposed obliquely in the path of the imaging light beam 14 for separating the imaging light beam 14 into a plurality of parallel, spatially separated, color component beams 40, 42, 44 (e.g., green, red and blue). Each layer 41, 43, 45 of the beam splitter 20 is adapted to reflect light in one spectral range and transmit light in other spectral ranges.
The color component beams 40, 42, 44 may be received by a photosensor 46 which may be comprised of a plurality of coplanar linear photosensor arrays 50, 52, 54. The linear photosensor arrays 50, 52, 54 are aligned with the color component beams 40, 42, 44, respectively. The color component beams 40, 42, 44 are focused on the linear photosensor arrays 50, 52, 54, respectively, at a common image plane II. The linear photosensor arrays 50, 52, 54 are typically charge coupled device (CCD) arrays adapted to transmit data signals representative of the intensity of the color component beams 40, 42, 44 to a suitable data processing and data storage unit (not shown) such as a personal computer or work station.
In order for the color component beams 40, 42, 44 to be focused at the image plane II, the distance travelled by each of the color component beams 40, 42, 44 from the imaging lens 12 to the photosensor 46 (also referred to herein as the "optical path length" or "total optical path length" of a beam) must be equal. It is apparent in FIG. 1 that, with this beam splitter configuration, the optical path lengths of the beams 40, 42, 44 are not equal. Thus, the color imaging assembly 10 must include a separate path length compensator such as the step-type path length compensator 60 shown in FIG. 1 which refractively compensates for differences in the optical path lengths of the color component beams 40, 42, 44.
Other techniques for resolving optical path length problems are disclosed in U.S. Pat. Nos. 4,709,144; 4,870,268; and 5,032,004 incorporated by reference above. For example, U.S. Pat. No. 5,032,004 discloses a beam splitter with three surface pairs, each pair comprising a surface which reflects light in two spectral ranges and a surface which reflects light in three spectral ranges. Color component beams created by the beam splitter are focused on an image plane which lies parallel to an associated photosensor array. The focus of the image provided by each color component beam on each photosensor array may be adjusted by varying the distance between the surfaces in each pair. Registration of each color component beam with an associated photosensor array may be controlled by varying the angular relationship between the surfaces in each pair. With this beam splitter design, the color component beams may cross one another and/or strike the image plane at an angle as opposed to being generally perpendicular to the image plane. This results in relatively weaker intensity beams being received by the photosensor array which causes the photosensor to transmit relatively weaker data signals to an associated data processing unit or the like.
A conventional multilayered dichroic beam splitter such as the beam splitter 20 of FIG. 1 is shown in more detail in FIG. 2. Such a beam splitter 20 is typically comprised of a thin glass plate 22 and a relatively thicker glass plate 24 which have a plurality of dichroic coatings 26, 28, 30 deposited thereon. Disposed between the plates 22, 24 is a spacer 23 which may be comprised of a thin glass plate having no coatings thereon. In particular, the thin plate 22 may be coated on a first side 32 with a dichroic coating 26 which reflects light in a first spectral range and transmits light in all other spectral ranges. The thin plate 22 may be coated on a second side 34 with a dichroic coating 28 which reflects light in a second spectral range and transmits light in all other spectral ranges. The thicker plate 24 may be coated on a first side 36 with a dichroic coating 30 which reflects light in a third spectral range and transmits light in all other spectral ranges. Layers of optical adhesive 38 may then be applied between the thin plate 22 and spacer 23 and also between the thicker plate 24 and spacer 23, and the plates 22, 24 and spacer 23 may be fixedly attached to one another.
Dichroic coatings are well-known in the art of optics and typically consist of 20-30 alternating high and low refractive index optical layers vacuum-deposited on a glass surface to an accumulative thickness of about 1-3 microns. The dichroic coating process must be performed under high temperatures, e.g., on the order of 200.degree.-450.degree. C., and the process usually takes approximately 8-12 hours to complete (including preparatory steps). Consequently, dichroically-coated glass is generally relatively expensive to produce. Thus, it would be desirable to minimize the total number of dichroic coatings used in a beam splitter.
Different types of coatings may be applied depending upon whether the surface to be coated will ultimately have an air immersed interface (i.e., wherein a glass surface interfaces with air such as at surface 32, FIG. 2) or a glass immersed interface (i.e., wherein a glass surfaces interfaces with another glass surface such as at surfaces 34 and 36, FIG. 2). Generally, the coating process for an air immersed interface is relatively less expensive and takes less time to complete than the coating process for a glass immersed interface. Furthermore, an air immersed surface coating process usually involves fewer coating layers, resulting in a thinner dichroic coating and fewer flatness problems than with a glass immersed surface coating. Thus, it would be generally desirable to minimize the number of glass immersed interfaces within a beam splitter.
For the multilayered dichroic beam splitter 20 of FIGS. 1 and 2 which is comprised of three dichroic coatings 26, 28, 30, the coating processes may take as long as 24-36 hours to complete. Also, two out of the three surfaces 32, 34, 36 each have a glass immersed interface and thus would require the less desirable glass immersed interface coating process. Furthermore, since one of the thin glass plates (e.g., the first plate 22) is coated on both sides 32, 34, the plate 22 would be subjected to the dichroic coating process twice, which presents several disadvantages as discussed below.
For instance, due to the number of coating layers (e.g., 20-30) as well as the high temperatures (e.g., 200.degree.-450.degree. C.) required in each coating process, a thin glass plate (e.g., 22) subjected to the coating process twice experiences a significant amount of stress which may cause considerable warpage, as shown in FIG. 3 with the warpage of plate 22 enhanced for illustrative purposes. Such warpage adversely affects the flatness of the thin glass plate 22 as shown and thus may result in color registration error. For example, an incident light beam 70 may deflect a first color component beam 72 from the first dichroic surface 82 of the warped plate 22, a second color component beam 74 from the second dichroic surface 84 of the warped plate 22, and a third color component beam 76 from the dichroic surface 86 of the flat plate 24. Proper color registration is achieved when the first, second, and third color component beams 72, 74, 76 are parallel. Due to the uneven surface of the warped plate 22, one or more of the color component beams 72, 74, 76 may be askew as illustrated in FIG. 3, resulting in color registration error.
Problems may also arise if a defect in the second dichroic coating 28 on the plate 22 is detected after the first dichroic coating 26 has been applied. In such a case, the entire plate 22 usually must be scrapped, and the time spent and expenses incurred in applying both dichroic coatings 26, 28 are wasted. Furthermore, the dichroic coating process for the second dichroic coating 28 may adversely affect the reflective properties of the first dichroic coating 26.
Thus, for the above reasons, it would be desirable to avoid the use of a multilayered beam splitter within a color imaging assembly.